In the wireless communications field, increasing use is made of MIMO (Multiple Input Multiple Output) circuits in order to increase the capacity of the transmission circuits and improve the coverage. The result is a use of a larger number of antennas. When the antennas are printed directly or placed on the printed circuit board or PCB, this induces, in the ground plane, currents that potentially affect the behaviour of the other antennas as well as that of the processing circuits positioned near the antennas. It is therefore necessary to isolate the elements from each other to limit current leakage without deteriorating the performances of the antennas. Moreover, in the case of MIMO systems, the antennas must be strongly isolated to obtain the level of decorrelation from the required signal to provide the optimum performances for the MIMO system. However, the space on the board being limited, the isolation constraints are very strong. In addition, in the case of systems operating at several frequency bands, an isolation between bands is most frequently required to ensure a correct coexistence of all the radios in the frequency band.
Moreover, shielding covers are frequently used fixed on the top of the processing circuit to reduce the electromagnetic interferences and radio frequencies, by reducing the radiation levels. This use is increasingly frequent as the current wireless systems are most often multi-band and multi-mode, which leads to greater interference problems. Moreover, the tendency toward miniaturization of wireless devices and toward the integration of multiple circuits on a same board requires improved isolation and shielding. In addition, the regulation relating to the levels of electromagnetic waves and radio frequencies is increasingly restrictive.
It is known, particularly in U.S. patent 2003/0193437, to use slots between two antennas to reduce the mutual coupling of the antennas and limits surface current leaks. However, this type of isolation requires surface because the length of the slots is noticeably equal to λ/4 where λ is the wavelength at the operating frequency of the circuit. Hence, for an operating frequency of 2.4 GHz, the wavelength is about 30 mm.